The present invention relates to the field of lighting, and in particular to an LED string driver with a power factor corrector and a current governor, the current governor providing prompt current limiting.
Solid state lighting, and in particular light emitting diodes (LEDs) are rapidly coming into wide use for lighting applications. In most general lighting applications the LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current.
LEDs providing high luminance exhibit a range of forward voltage drops, denoted Vf, and their luminance is primarily a function of current. Brightness control of the LEDs may be performed by either pulse width modulation (PWM) or by amplitude modulation. In a PWM brightness control a fixed current is driven through the LED string, and the duty cycle of the fixed current is adjusted in order to control the LED string brightness. In amplitude modulation the amount of current through the LED string is varied directly, thus adjusting the brightness.
The power factor of an alternating current (AC) electric power system is defined as the ratio of real power to the apparent power flowing to a load. Real power is the capacity of the circuit to perform work in a particular time, whereas apparent power is a product of the current and voltage of the circuit. Power is lost in the system when the power factor is significantly below unity. A power factor corrector (PFC) may be advantageously utilized to control the power source providing electrical energy to the LED string so as to achieve a power factor approaching unity. A power factor corrector typically comprises an error amplifier and a multiplier arranged to cooperate so as to maintain a high power factor while controlling a power converter so as to converge the input to the error amplifier towards a reference value. The bandwidth of the error amplifier loop is constrained to be less than twice the AC line frequency, typically less than 20 Hz, so that the error amplifier's output is relatively constant over a given AC line cycle.
LED strings exhibit a particular voltage to current relationship, wherein for a voltage below a minimum operating voltage no appreciable current flows, and for voltages exceeding the minimum operating voltage the current follows an exponential curve responsive to the voltage. Small changes in voltage thus result in very large changes in current, which may result in extremely large power surges before correction by the slow response time of the PFC control loop.
A two stage power source and driver provides a first stage with PFC and a second stage which advantageously exhibits a fast control loop, capable of preventing such large power surges. Unfortunately, a two stage power source and driver adds expense and may further exhibit a reduced efficiency as compared with a single stage power source and driver.